1. Field of the Invention
The present invention relates generally to high-power microwave switches.
2. Description of the Related Art
Microwave switches typically transition between a transmissive state and a reflective state in response to a control parameter. The choice of control parameter is related to the intended use of the microwave switch.
For example, transmit/receive (T/R) switches are typically used in radar systems to protect a radar receiver from reflected signals of high-power transmitter pulses. In this application, it is imperative that the control parameter is the reflected pulse itself. Thus, T/R switches are generally designed to change from their transmissive state to their reflective state in response to an incident microwave signal that exceeds a predetermined threshold.
In contrast, a microwave switch for directing microwave signals in a microwave network must respond to an external trigger signal. Preferably, a triggered microwave switch for network use exhibits a low insertion loss in its transmissive state, reflects a signal having high phase stability when in its reflective state and transitions quickly between the two states.
Although prior work on triggered microwave switches has not been as extensive as the work on T/R-type switches, a variety of triggered switches have been developed. For example, U.S. Pat. No. 3,611,008 discloses an exemplary triggered waveguide switch which includes a pair of main electrodes and a trigger electrode. The main electrodes are composed of a low vapor pressure metal, e.g., copper, and are separated to form an electrode gap. The main electrodes are either positioned within an evacuated waveguide section or within a chamber that communicates with the waveguide section. The material of the trigger electrode, e.g., titanium hydride, contains a stored gas, e.g., hydrogen, and the trigger electrode is spaced from one of the main electrodes.
In operation, a potential is placed across the main electrodes and a voltage pulse applied to the trigger electrode. The pulse initiates a spark whose discharge energy releases and ionizes a portion of the stored gas. This reduces the dielectric strength in the main-electrode gap which induces an arc between the main electrodes. Metal ions are boiled off the electrodes and ionized to form a plasma which fills the waveguide section and reflects incident microwave signals. The plasma will be maintained as long as the main electrode potential is sustained. Unfortunately, the metal vapor tends to collect on the waveguide windows which increases the insertion loss of the waveguide switch when it is in its transmissive state. Although this problem can be reduced by introducing waveguide septums to block the flow of metal ions to the waveguide windows, the septums also increase the switch's insertion loss.
Another exemplary triggered microwave switch is described in U.S. Pat. No. 3,903,489. This switch has a waveguide section which is filled with a low-pressure controlled atmosphere which is suitable for supporting a glow discharge. A plasma generator includes an anode and a control grid which form opposite sides of the waveguide section but are electrically isolated from the remainder of the waveguide. This arrangement concentrates the anode's electric field in the waveguide section so that most of the field is available to accelerate electrons which reach the vicinity of the control grid. In operation, a high-density plasma is injected into the waveguide section by the anode's electric field. This places the waveguide section in a high insertion loss state so that an incident microwave signal is substantially reflected. The plasma is triggered by a trigger pulse which is applied between the control grid and the anode. The power to keep the waveguide section in its high insertion loss state is supplied by the plasma generator.
As shown by these examples, triggered microwave switches have been developed but they are typically complex (e.g., U.S. Pat. No. 3,611,008 describes main electrodes, a trigger electrode and isolating septums and U.S. Pat. No. 3,903,489 describe heater, cathode, control grid, anode and focusing structures), have elements which typically have a short lifetime (e.g., the low vapor pressure electrodes of U.S. Pat. No. 3,611,008 and the heater of U.S. Pat. No. 3,903,489) and require significant input power (e.g., the main electrode potential of U.S. Pat. No. 3,611,008 and the plasma generator of U.S. Pat. No. 3,903,489).